Develop Reference

OpenSilver and Dispatcher.CheckAccess

I am working on porting an old Silverlight application over to OpenSilver. Throughout the Silverlight code there are if( <control>.CheckAccess())... to make sure to be on the correct thread. Is my impression this check is no longer needed in OpenSilver? In other words, the following Silverlight code can be transformed into the following:
Yes, I know that callback-based async methods have been replaced with awaitable tasks. I am going to ask some questions about that conversion in my next question, here. This question is exclusively about the fate of the Dispatcher.CheckAccess
Silverlight:
private void GetNextImage()
{
var cmc = ServiceFactories.CreateCartManager();
cmc.getSlideImageCompleted += (s, e) =>
{
if (imageGrid.CheckAccess())
{
cmc_getSlideImageCompleted(s, e);
}
else
{
var args = new object[] { s, e };
imageGrid.Dispatcher.BeginInvoke(new getSlideImageCompletedDelegate(cmc_getSlideImageCompleted),
args);
}
};
var lastTime = SystemSettings.GetInstance().SlideShowData.LastImageTime;
cmc.getSlideImageAsync(string.IsNullOrEmpty(lastTime) ? null : lastTime);
}
to OpenSilver:
private void GetNextImage()
{
var cmc = ServiceFactories.CreateCartManager();
cmc.getSlideImageCompleted += (s, e) =>
{
cmc_getSlideImageCompleted(s, e);
};
var lastTime = SystemSettings.GetInstance().SlideShowData.LastImageTime;
cmc.getSlideImageAsync(string.IsNullOrEmpty(lastTime) ? null : lastTime);
}

There is no need to use Dispatcher.CheckAccess since OpenSilver is currently single threaded (it uses mono.wasm runtime which doesn't support threads yet).
However, OpenSilver keeps compatibility with Silverlight, so if you have an old Silverlight code which does the check you can just keep it (it will always return true when running in Browser).

Close MessageBox when incoming new message

i created simple application to send and receive message using C# and GsmComm Library. if there is a new incoming message my application will show messageBox that new message arrived. my problem is when i have many new message, messageBox will show so many messageBox notication. How can I just show the last single message box using code?
this mycode:
private void comm_MessageReceived(object sender, MessageReceivedEventArgs e)
{
var obj = e.IndicationObject;
if (obj is MemoryLocation)
{
var loc = (MemoryLocation)obj;
var msg = string.Format("New message received in storage \"{0}\", index {1}.",
loc.Storage, loc.Index);
MessageBox.Show(msg);
return;
}
}
i confuse to fix this, i tried to another way using form to show new incoming notif form1.showDialog(); but same problem first form show cannot be closed when new form opened. this my reference: https://stackoverflow.com/a/13445167/3319555
I really thanks if anyone can help me..thanks

If you're using your second solution of displaying a form with form.ShowDialog() you can store the forms in a list. Then, when a new form needs to be displayed, you can iterate through the list and close each open form with form.Close(). Assuming that your comm_MessageReceieved method is run on another thread, which I assume is driven via an IO completion port, then something like this perhaps?
List<MyForm> formList = new List<MyForm>();
readonly object formListLock = new object();
private void comm_MessageReceived(object sender, MessageReceivedEventArgs e)
{
/// you need to lock the List for thread safe access
lock (formListLock)
{
/// iterate over a copy of the list to avoid mutating the list under iteration
foreach (MyForm form in formList.ToList())
{
form.ThreadSafeClose();
}
}
string msg = "message";
using (MyForm form = new MyForm(msg))
{
lock (formListLock) { formList.Add(form); }
form.ShowDialog();
lock (formListLock) { formList.Remove(form); }
}
}
This was just off the top of my head but might be another possible direction you could take.
You will have to make a thread safe call to form.Close() so that it is run on the form's UI thread. Read about invoke here. There's a lot of information on SO about this topic. This could be as simple as adding something like the following method to your form class:
public void ThreadSafeClose()
{
if (this.InvokeRequired)
{
this.Invoke(new Action(Close)); /// or BeginInvoke...
}
else
{
Close();
}
}
Read more about Lists here: https://msdn.microsoft.com/en-us/library/6sh2ey19%28v=vs.110%29.aspx
Read more about the lock statement here: https://msdn.microsoft.com/en-us/library/c5kehkcz.aspx
Read more about thread synchronisation here: https://msdn.microsoft.com/en-us/library/ms173179.aspx
There are also numerous thread-safe collections that could possibly suit your needs, e.g. ConcurrentBag.

Visual Studio during Debugging: The function evaluation requires all threads to run

I'm suddenly getting a strange error while debugging. Up to now the variable in the watch windows has been showing correctly. Now I am always getting this error message in the watch windows:
The function evaluation requires all threads to run
I am not able to check any variable anymore. I am not explicitly working with threads. What can I do to get it working again?
I already disabled, as mentioned in some forums, the function: "Enable property Evaluation and other implicit function Calls" in the option window of the debugger. But without success, and it gives me this error:
Error Implicit Function evaluation disabled by the user

From the msdn forum:
This isn't an error in and of itself, but more of a feature of your debugger.
Some properties require code to be executed in order for the property to be read, but if this requires cross-thread interaction, then other threads may have to run as well. The debugger doesn't do this automatically, but certainly can, with your permission.
Just click the little evaluate icon and it will run your code and evaluate the property.
For further details on this behaviour check this excelent article

I ran into this issue when just trying to get items from a table called "AGENCY" using Entity Framework:
var agencies = db.AGENCY.OrderBy(e => e.FULLNAME);
Hovering over agencies in debug mode, clicking to expand the options, and clicking Results would give the dreaded "The function evaluation requires all threads to run" with a "Do Not Enter" icon at the end that, on which, clicking did nothing.
2 possible solutions:
Add .ToList() at the end:
var agencies = db.AGENCY_TABLE.OrderBy(e => e.FULLNAME).ToList();
List<AGENCY_TABLE> agencies = db.AGENCY_TABLE.OrderBy(e => e.FULLNAME).ToList();
Credit goes to Hp93 for helping me come to this solution. In the comments on MUG4N's answer where I found this solution, it also mentions trying .Any() instead of .ToList(), but this gives a Boolean instead of a <T>, like <AGENCY> is, so it probably wouldn't help.
Workaround - try a different path in the debug options. I found that I could click on the "Non-Public Members" > "_internalQuery" > ObjectQuery > Results View and get my values that way.

MUG4N has indeed provided a correct answer however if you hover over the line of code in debug, you may be looking at something like the below. If so, click the little re-evaluate icon highlighted in the image below...
NB: I obtained this image by pinning, normally the re-evaluate icone are in the middle of the window and not down the left hand column.

You should make thread safe call because accessing Windows form controls are not Thread safe in multithreading.
This is my simple code which makes Thread safe call and sets Progress bar.
public partial class Form1 : Form
{// This delegate enables asynchronous calls for setting
// the text property on a TextBox control.
delegate void StringArgReturningVoidDelegate(string text);
private Thread demoThread = null;
public int Progresscount = 0;
static EventWaitHandle waithandler = new AutoResetEvent(false);
public Form1()
{
InitializeComponent();
}
public static bool CheckForInternetConnection()
{
try
{
using (var client = new WebClient())
{
using (var stream = client.OpenRead("http://www.google.com"))
{
return true;
}
}
}
catch
{
return false;
}
}
public void Progressincrement()
{
waithandler.WaitOne();
while (CheckForInternetConnection()==true)
{
if (Progresscount==100)
{
break;
}
SetLabel("Connected");
Progresscount += 1;
SetProgress(Progresscount.ToString());
Thread.Sleep(TimeSpan.FromSeconds(1));
}
if (Progresscount <100)
{
Startthread();
}
SetLabel("Completed");
}
public void Startthread ()
{
this.demoThread= new Thread(new ThreadStart(Progressincrement));
this.demoThread.Start();
SetLabel("Waiting for connection");
while (CheckForInternetConnection() == false) ;
waithandler.Set();
}
private void SetLabel(string text)
{
// InvokeRequired required compares the thread ID of the
// calling thread to the thread ID of the creating thread.
// If these threads are different, it returns true.
if (this.label1.InvokeRequired)
{
StringArgReturningVoidDelegate d = new StringArgReturningVoidDelegate(SetLabel);
this.Invoke(d, new object[] { text });
}
else
{
this.label1.Text = text;
}
}
private void SetProgress(string Value)
{
// InvokeRequired required compares the thread ID of the
// calling thread to the thread ID of the creating thread.
// If these threads are different, it returns true.
if (this.progressBar1.InvokeRequired)
{
StringArgReturningVoidDelegate d = new StringArgReturningVoidDelegate(SetProgress);
this.Invoke(d, new object[] {Value});
}
else
{
this.progressBar1.Value = Convert.ToInt32(Value);
}
}
private void Form1_Load(object sender, EventArgs e)
{
Startthread();
}
private void button1_Click(object sender, EventArgs e)
{
MessageBox.Show("Responsive");
}
}
For more information MSDN

This isn't an error, but more of a feature of your debugger.
The debugger doesn't do this automatically, but certainly can, with users permission. Just click the little space icon and it will run the code and evaluate the property.

I use the next workaround to pass:
var OtherThreadField = "";
Invoke(new MethodInvoker(delegate
{
OtherThreadField = ExecuteNeededMEthod();
}));
Now i have a value for OtherThreadField.

I faced the same issue and solved .The Issue arise due to username and password ,in SQL connection there is user and password but in code there no user and password. so I enable the user and the password and the issue solved

For me, this happened when trying to break on a line that accesses a complex object instance contained by a Settings Class.
A breakpoint on the following if results in Settings.Default.FindSettings with the value being "The function evaluation requires all threads to run." If I press the force eval button, it is null. Stepping with the force eval button click or not enters the if block and initializes the object. If I remove the breakpoint and add a new breakpoint following the if block, the Settings.Default.FindSettings deserializes properly with the expected values.
if (Settings.Default.FindSettings == null)
{
Settings.Default.FindSettings = new FindSettings();
}
After trial and error, I added the following code before the above if block to access the settings prior to breaking. This seems to reliably fix the problem. I do not need it in production so I wrap in conditional compiler directive. I have a comment in the code instead of a non-descript discard:
#if DEBUG
var _ = Settings.Default.FindSettings;
#endif
I am not sure if the above line would be optimized out in production since it has side effects. As I only need it while debugging, I have not checked.

Removing Text with an Invoke?

So, the documentation that I've found online so far regarding the Invoke property doesn't seem to be particularly good, which is actually kind of annoying, believe it or not. I understand what Invoke does - you can't safely access the GUI directly, so an invoke does it in a safe way. That's fine, but I don't understand the variables that go into the method. If I wanted to, for instance, remove text from a listbox, how would I do that? I get about this far before I get a bit lost.
private void DoStuff(string TextIWouldBeRemoving)
{
if (listboxname.InvokeRequired)
{
listboxname.Invoke(SomeMysteriousParamaters, new object[] { TextIWouldBeRemoving )};
}
}

The first parameter is the method you want to safely invoke, the second parameter is an object array of the arguments to that method
So you would write:
private void DoStuff(string TextIWouldBeRemoving)
{
if (listboxname.InvokeRequired)
{
listboxname.Invoke(DoStuff, new object[] { TextIWouldBeRemoving )};
}
else
{
// Actually remove the text here!
}
}

Invoke is all about threading.
You need to do an invoke whenever you have created a separate thread in your code, and you need to update the User Interface elements from withing the code, that is executing in that newly create thread.
You can use a BeginInvoke, instead of a synchronous Invoke method. This article has a good example:
http://msdn.microsoft.com/en-us/library/0b1bf3y3.aspx

private void button1_Click(object sender, EventArgs e)
{
if (listBox1.InvokeRequired)
{
Action<string> d = DoAnything;
listBox1.Invoke(d, new object[] { "Item 1" });
}
else
DoAnything("Item 1");
}
void DoAnything(string itemText)
{
listBox1.Items.Remove(itemText);
}

Accessing Class members with Invoke from a different thread in C#

Note: Part of a series: C#: Accessing form members from another class and How to access form objects from another cs file in C#.
Hello,
The Idea is to notify the user using the memo when a packet is received/sent in a TCP Client.
After couple of fixes,the most suitable solution seemed to be this one
public string TextValue
{
set
{
this.Memo.Text += value + "\n";
}
}
That's how it's being called
var form = Form.ActiveForm as Form1;
if(form != null)
form.TextValue = "Test asdasd";
However,calling the code throws an exception ,because of Unsafe thread call.I found a solution at msdn,but I can't seem to acquire the method they've used there.
This is my remake,which doesn't work.
private void SetTextMemo(string txt)
{
if(this.Memo.InvokeRequired)
{
this.Invoke(SetTextMemo,txt); //error here
}
else
{
this.Memo.Text += txt + "\n";
}
}
errors:
Argument '1': cannot convert from 'method group' to 'System.Delegate'
Argument '2': cannot convert from 'string' to 'object[]'
Basically,I'm trying to access the Memo(or more likely said,add text to the memo) from another thread using Invoke.I never used it before,maybe that's why I misunderstand my mistake.

The easy way is:
this.Invoke((MethodInvoker)delegate {
this.Memo.Text += txt + "\n";
});
Which uses an anonymous method to do the job inline. Since you expect to be on another thread, you may as well just call Invoke - it is safe even from the UI thread.

If you're using C# 3.0 and the 3.5 framework try the following
if ( this.Memo.InvokeRequired ) {
this.Invoke((Action)(() => SetTextMemo(txt)));
}

Your implementation assumes that the method will not infinitely recurse because the behavior of the InvokeRequired property will prevent it. This assumption may proove to be true, but there's no problem coding the function to avoid this possibility entirely. Here's what I suggest:
private void SetMemo(string txt)
{
Memo.Text = txt;
}
private delegate void MemoSetter(string txt);
public void ThreadSafeSet(string txt)
{
Invoke(new MemoSetter(SetMemo), txt);
}

I used to handle all this cross-thread business, but recently I went with AOP, where you simply decorate a method to execute on the UI thread. Here's an example (from PostSharp):
public class FormsThreadAttribute : OnMethodInvocationAspect
{
public override void OnInvocation(MethodInvocationEventArgs eventArgs)
{
Form f = (Form)eventArgs.Delegate.Target;
if (f.InvokeRequired)
f.Invoke(eventArgs.Delegate, eventArgs.GetArgumentArray());
else
eventArgs.Proceed();
}
}